1. Field of the Invention
The present invention relates to scanning radio receivers. More particularly, the present invention relates to control systems for optimizing the efficiency of reception of intermittent audio content broadcast on plural channels and received by plural frequency agile receivers.
2. Description of the Related Art
Scanning radio receivers, commonly known as “police scanners” or simply “scanners”, allow users to listen to police, fire, aircraft, marine, business and other manner of one-way and two-way radio communications across a broad spectrum of frequencies, typically from 25 MHz to 1300 MHz, and including higher frequencies as well. Scanners typically have a channel memory that is used to store one or more receiver frequencies, or indicia of frequency, which can be recalled by referencing a channel number, thereby simplifying the entry and selection of desired reception frequencies. Various types of scanners are known, some operating in a few bands of frequencies with limited channel memory capacity, others being full-featured models that cover all the pertinent bands and including generous channel memory capacity. Scanners are enabled to sequentially change reception frequencies, thereby scanning through a list of frequencies, searching for broadcasts that may or may not be of interest to a user. In modern scanners the selection of a radio frequency generally includes specifying the radio frequency band and the receiver phase-locked loop (PLL) divisor that is requires to tune the receiver to discriminate the precise frequency of interest. Thus, the specification of the RF-band and PLL divisor with a digital selection means enables a precise reception frequency in most scanners. Modern radio broadcast systems employ digital processor to control the allocation of frequencies for radio communications. This typically also include spectrum utilization efficiency improving techniques that enable systems to offer a greater number of communications “channels” than the number of actual radio frequencies that may exist in a system. Thus, a single frequency may be utilized for a large number of channels, which are managed by the system protocol. The system protocols use various techniques for defining and allocating channels, and modern scanners have corresponding decoding systems or distinguishing channels from one another, as is appreciated by those skilled in the art.
Scanner radio receivers typically employ some form of squelch control so that noise and undesirable communications are not routed through to a loudspeaker or other audio output. Carrier squelch can be used, which gates received audio to a loudspeaker based on the signal-to-noise ratio or carrier-to-noise ratio of the receiver demodulator output. Other systems employ out of band tones that are detected to control squelch. This is an example of a technique to provide more than one channel of communications on a single frequency. Certain receivers are sensitive to certain tones, and therefore communication that only those channels. One such system employs plural sub-audible tones, and is referred to as a continuously tone coded squelch system (“CTCSS”). The receiver checks for a particular one of the plural tones based on the channel programming, and detection of a matching tone enables the squelch gate of the receiver. Another system employs digital data fields that are broadcast along with the primary communication signals, and the receiver looks for a matching digital code. Such systems are referred to as digitally coded squelch systems (“DCS”). Other squelch control systems are known as well.
Early two-way radio systems employed a single radio frequency or a duplex pair of radio frequencies for two-way communications. Such systems lent themselves well to scanner receiver monitoring because a given two-way radio fleet of users, such as the local police department, could be readily monitored by receiving a single, predetermined, radio frequency. However, heavy radio use demand and congested airways caused manufacturers to develop more spectrally efficient radio systems. One solution was the trunked radio system where a group of two to twenty-eight duplex pairs of radio frequencies are allocated together for shared use by plural fleets of users. In a trunked system, talk group identities are assigned to the fleets, which are used to provide receiver squelch gate control for the plural members of the fleet. The difference in a trunked radio system vis-à-vis a conventional system is that the radio frequencies are dynamically allocated during use. As such, a conversation between a dispatcher and a fleet of patrol cars, for example, can change from frequency to frequency within the trunked group of frequencies during the course of a conversation. Suppliers of scanning receivers addressed this difference in functionality by developing radios that could track the talk group identities (“Talk group ID's”) and dynamically hop from frequency to frequency as the conversation progressed. The key to radio scanner operation in a trunking environment is to have all of the trunking frequencies for each trunk group stored in the scanner channel memory, typically associated with a system identity (“System ID”), and then track the talk group ID code of the desired fleet, along with the dynamic allocation of the trunking frequencies. In this way, the trunked scanner functions like a conventional scanner from the user's perspective, with the “channel” actually associated with both a trunking system ID and a talk group ID instead of the conventional radio system frequency-to-channel, plus squelch code, correlation. Certain trunking systems dedicate one of their allocated frequencies as a control channel carrying relatively high speed data signals, which are monitored by receivers looking for assignment to a talk channel from time to time.
Two-way mobile radio communications systems are widely used for a variety of applications including public safety, commercial, and personal communications. Radios with transmission and receive elements, commonly known as transceivers participate in these communications. In addition, radio receivers monitor communications without participating through transmissions.
Most two-way radio communications operate according to a transmission trunked control systems. This is different from the conversation trunked system. For example, a PSTN telephone call is conversation trunked in that the communications resource is set-up and maintained for the entire duration of a conversation, even during periods of quiet between the parties to the conversation. In a transmission trunked environment, the system operates in a push-to-talk mode. In this situation, each verbal statement from each user is individually transmitted. Each statement by a party to a conversation is transmitted on a radio frequency, and a conversation usually comprises a series of separate transmissions with periods of quiet in which no radio signal occur between individual transmission signal elements. In some instances, if the gap between remarks is short, a transmitter may remain active with no gap in the carrier signal of the radio transmission between remarks. When used for conversation, the result is a radio channel with a series of separate transmissions, each with a relatively short duration, typically in the range of two to sixty seconds. Depending on the level of activity on a channel, this may generate a regular patter of activity, or there may be inactive gaps extending to hours between conversations.
For the sake a clarity, the terms ‘channel’, ‘frequency’, ‘signal’ and ‘squelch’ are used as follows. The term ‘channel’ refers to a discrete communications path for the transmission of certain classes of related content that may be independently identified at a radio receiver, regardless of whether this path is currently active with the presence signal or inactive without the presence of signal, such as a radio broadcast frequency, a coded squelch broadcast signal, or trunked radio system talk group ID. The term ‘frequency’ refers to an actual radio broadcast frequency on which a communications signal is modulated or may be modulated, such as a conventional frequency or a trunked system working channel. The term ‘signal’ refers to a discrete period of activity on a channel, such as a single radio transmission, or a series of closely spaced but discrete transmissions. In some cases, evident by context, ‘signal’ may refer to the content currently present on a broadcast frequency. The term ‘squelch’ refers to a test determining whether signal is present on a particular frequency; squelch is true when there is no signal, and unsquelch is true when there is signal.
In common speech there may be confusion between these terms. For instance, in conventional radio systems, there is typically a one-to-one regional correspondence between channels and locally active broadcast frequencies. This encourages a perceived equivalence between the terms, or blurring of meanings. However, the terms have different technical meanings herein.
Frequency agile receivers, commonly known as “scanners”, are designed to receive signals on multiple communications channels by sequentially sampling (“scanning”) predetermined channels until an active signal is detected, and holding on that channel to receive audio until the transmission or series of transmissions is complete. The scanner then resumes the scanning process to detect the next new signal. Typical scanners can scan hundreds of channels. Since any individual channel will typically have long periods of inactivity, this technology is a practical way to monitor communications on multiple channels with a single receiver, although it is typically not plausible to receive simultaneous communications on different channels with a single receiver.
When a single receiver is receiving a transmission on one channel, it is typically insensitive to any radio communications on other channels. As a limited exception to that principle, some scanners have a ‘priority’ feature wherein the scanner periodically retunes to a designated “priority” frequency to test for signal, at the cost of brief gaps in the reception of the present non-priority signal. This tradeoff provides for greater reliability in coverage of signals on the designated priority frequency, at the direct cost of performance in the reception completeness of all non-priority signals.
Since a single receiver can only actively receive a single signal at a time, it will miss some fraction of the total communications activity of the collective set of channels being monitored, the so called scan list. The missed fraction depends on factors such as the average fraction of time that channels are in use, and the number of such channels in the scan list. In general, continuity of reception is desirable, and missed transmissions represent a deficiency in the system performance.
It is normal for unrelated transmissions using different channels to occur at the same time. For some applications, it is advantageous to detect and receive such simultaneous radio transmissions. For example, this is important for logging systems that preserve content for archival purposes. It is also important in dispatch and newsroom environments where there is no foreknowledge about which particular transmissions will convey information of importance. A prior art solution is to operate a permanently dedicated radio receiver on each desired radio channel. For instance, when monitoring 30 channels, an array of 30 radio receivers can provide complete reception coverage. However, for large numbers of channels, operating an equal number of receivers can become prohibitively expensive in terms of both electrical equipment and physical housing requirements. Therefore, despite the theoretical appeal of this simple solution, there exists a need for a more practical solution to this radio monitoring scenario, especially when trying to receive more than a few channels.
In the special case of trunked radio systems, where the content of a larger number of logical channels is always found on a smaller number of broadcast working frequencies, and which are indexed in accordance with data transmitted on an additional control channel frequency, it is possible to operate a fixed-frequency single receiver on each operational frequency within the trunked radio system, and thereby capture all content without recourse to a separate receiver for each logical channel. For example, this is taught in U.S. Pat. No. 5,710,978 to Swift for Logging Recorder System for Trunking Radio. However, a technique built upon permanently dedicated receivers becomes impractical due to the proliferation of separate frequencies to monitor when the channels to monitor include channels not carried by a particular trunked radio system.
Note that Swift uses the term “channel” in the sense of a trunked radio system “working channel”, to refer to a specific broadcast frequency irrespective of the “logical channel” content carried thereupon at a given time. This use of “channel” is distinct from reference to a “logical channel” wherein each channel by definition refers to a key value indicating that content thereupon is related, even if carried upon different broadcast frequencies.
Due to the likelihood of missing transmissions on a single receiver, some users operate multiple scanning radios in parallel. However this method has serious shortcomings that limit its practical utility for comprehensive detection and reception of active signals on multiple channels. For example, consider the situation where a single channel is included in the scan list of multiple receivers. Then, whenever there is a signal on that channel, it is entirely possible that all of the receivers will detect that signal, and remained tuned to that channel until the end of transmission. This behavior is expected whenever, at any time during the period of active signal, there is no activity on the other channels being scanned by these receivers. As long as these receivers remain stopped on that initial signal, then new signals on other channels that begin during the remainder of the first signal transmission will be missed by all of the receivers. During a period when multiple receivers are monitoring the same signal, the intended benefit of using multiple receivers for more complete reception coverage is lost. This renders ineffective the option of programming a group of scanners identically, with the scan list of all desired channels.
Reference is directed to FIG. 3, FIG. 4, and FIG. 5, which together illustrate the aforementioned problem in the prior art of scanning multiple channels with plural receivers that are all programmed to scan the same channel list. FIG. 3 illustrates exemplary signal activity on five channels that are labeled A, B, C, D, and E. Time passes through intervals labeled t1, t2, t3, t4, t5, t6, t7, and t8. Note that the signals 24 are transmission trunked and sporadic in nature. FIG. 4 illustrates a reception timing diagram of three scanning radios labeled R1, R2, and R3 that are all programmed to scan all of the channels A, B, C, D, and E illustrated in FIG. 3. As a practical matter, transmission trunked signal segments 24 range in duration from a few seconds and longer. On the other hand, the channel scan rate of modern scanning radios is a few milliseconds. Thus, for this exemplary analysis, it can be assumed that the scan rate is virtually instantaneous since there are just five channels on the scan list. Thus, what FIG. 4 demonstrates is that all five radios A, B, C, D, and E received exactly the same channel signals by reason of the sequence in which the signals appear in time. Note that the long duration signal on channel C is captures by all of the radios. Now, if the captured signals in FIG. 4 are subtracted out from the total signals 24 in FIG. 3, what remains are the un-captured singles. The un-captured signals are illustrated in FIG. 5.
While use of a priority feature arguably creates a minor exception to the foregoing general rule, the same outcome is found once the scan list includes two channels not present on other receivers. In priority mode, a receiver periodically tunes away from an active signal and tests for activity on a priority channel. It is an intrinsic disadvantage of priority mode operation that an ongoing reception must me periodically interrupted to check the priority channel. If such a signal is found, the radio will remain tuned to the priority signal. Otherwise, the receiver will resume monitoring the original signal. If a channel is included in the scan list of a single receiver, but marked as the priority channel for that receiver, then all transmissions will be captured on that channel regardless of the receiver's other activity. This is a prior art method for ensuring complete reception in the case of a limited number of channels to be scanned where the number does not exceed the number of available radio receivers. However, whenever the number of monitored channels is greater than the number of available radio receivers, as is typically the case when scanners are used, since otherwise it would be possible to have a single dedicated receiver for each channel, this approach becomes ineffective for ensuring effective coverage of the entire set of channels even if the number of active channels is typically smaller than the number of available radio receivers.
A more practical prior art approach is to divide a given scan list of channels into discrete subsets, and program each receiver with one of these subsets. For example, if monitoring 50 channels with 5 receivers, each receiver may be programmed to scan 10 of these channels, with no overlap. Avoiding any channel duplication in the multiple receivers eliminates the problem of multiple receivers locking on the same signal, which would clearly reduce the effectiveness of having multiple receivers. However, if there is simultaneous activity on two of the channels in the scan list of a any individual receiver, then one of these signals will be missed, even if one or more of the other receivers are scanning and not presently monitoring any active signals.
For many applications, it is highly desirable for reception of a channel over a period of time to maximize continuity, that is, to reduce the rate of missed transmissions to zero, or as close to zero as practical. The phenomenon of missed transmissions presents a problem for any users who wish to comprehensively monitor traffic on a significant number of channels. Such a capacity would be beneficial for many applications. A first example is archival recording or logging of radio traffic, to provide a ‘black box’ record of events for later analysis in the event of an incident. A second example is feeding audio into an audio distribution network, so users of the network can independently select and monitor all traffic on specific channels of interest on demand, without providing these users with the capacity or authority to reprogram or direct a receiver so it remains tuned to a channel. A third example is for use in a dynamic environment where users want to have instant access to complete coverage of multiple audio sources, without the delay or risk of error associated with altering the programming of receivers. The ability of users to perform these functions is severely hampered by the lack of a technology in the prior art for effectively utilizing a group of multiple radio receivers to dynamically receive up to the same number of simultaneous signals, regardless of the particular set of channels active at that time. Thus, it can be appreciated that such a need exists.